Cellular Morphogenesis in Plants - Mechanisms of Morphogenesis - page 58

FIGURE 6.2 A bias in the direction of tension-bearing microfibrils causes directional expansion in response to
internal pressure. In the interests of simplification, this diagram shows all fibrils orientated one way (in practice, the
bias will not be absolute, lest the cell burst) and depicts the cell as round (most plant cells are cuboidal, as depicted
in the rest of the diagrams in this chapter).
Detailed examination of the alignment of cellulose microfibrils in actively elongating plant
cells has confirmed that they are indeed aligned predominantly at 90 degrees to the axis of
expansion. 4,5 If cellulose walls are taken away, expansion is isotropic. 6 According to one
popular model, the problem of setting the direction of plant cell elongation therefore reduces
to that of setting the direction of cellulose microfibrils, which in turn reduces to that of setting
the direction in which the cellulose synthase complex moves through the plasma membrane.
In cells that are undergoing elongation, there is a clear anatomical correlation between the
direction in which cellulose microfibrils are laid down outside the plasma membrane and the
direction in which microtubules run just inside the plasma membrane. 7 The correlation can
also be demonstrated functionally, by mutations in the angiosperm Arabodopsis thaliana. The
fass mutation results in severe disruption to the organization of the cortical microtubule cyto-
skeleton 8 and is associated with defective cell elongation. Similarly, the spiral1 mutation
causes microtubules to line up obliquely rather than transversely across the cell and creates
a matching defect in elongation. 9 Unfortunately, the molecular basis of neither of these muta-
tions is understood, so that the correlation between orientation of microtubules and cellulose
fibrils remains just a correlation, with no indication of the direction of cause and effect.
Oblique alignment of microfibrils may, however, be a 'default' pattern when guidance fails:
it is seen when elongation ceases in normal development, 7 as well as when mutations or
drugs damage the microtubule system, 9 and once normal cells produce oblique microfibrils.
This happens when elongation has ceased and cell walls are thickening; the relationship
between the orientation of microfibrils and that of microtubules seems to be lost. 9,10
There is both morphological and biochemical evidence that microtubules determine the
orientation of cellulose microfibrils. The morphological evidence is that, in at least some plant
cells, microtubules are aligned more precisely with each other than are microfilbrils. 10 This